An acceleration sensor that employs a piezoelectric element is a known example of an acceleration sensor of the related art. For example, the below-cited Patent Document 1 describes an example of such an acceleration sensor. A schematic sectional view of the acceleration sensor described in Patent Document 1 is illustrated in FIG. 3.
As illustrated in FIG. 3, an acceleration sensor 100 includes a supporting member 101 and a piezoelectric element 110 that is arranged inside the supporting member 101 and has an end portion thereof clamped by the supporting member 101. The piezoelectric element 110 includes a piezoelectric substrate 111 and first and second electrodes 112a and 112b that are provided inside the piezoelectric substrate 111 and on surfaces of the piezoelectric substrate 111 and that oppose each other in the thickness direction of the piezoelectric substrate 111.
When an acceleration acts on the acceleration sensor 100, the piezoelectric element 110 deforms in accordance with the acceleration. As a result, a voltage is generated between the first and second electrodes 112a and 112b that has a magnitude that corresponds to the magnitude of the applied acceleration. The acceleration can be detected by measuring the magnitude of this voltage.
In the acceleration sensor 100, portions of the piezoelectric substrate 111 that are not clamped by the supporting member 101 and in which the first and second electrodes 112a and 112b oppose each other in the thickness direction are polarized. The end portion of the piezoelectric substrate 111 that is clamped by the supporting member 101 is not polarized. Consequently, the acceleration can be detected with high accuracy even when the temperature of the atmosphere in which the acceleration sensor is arranged changes.
That is, when the temperature of the atmosphere in which the acceleration sensor is arranged increases, heat is transmitted to the portion of the piezoelectric substrate that is clamped by the supporting member via the supporting member. Consequently, when the portion of the piezoelectric substrate that is clamped by the supporting member is polarized, charge is generated due to the pyroelectric effect and a voltage that is higher than a voltage corresponding to the magnitude of the acceleration applied to the acceleration sensor is extracted from the acceleration sensor. Therefore, the acceleration cannot be detected with high accuracy.
With respect to this, in the acceleration sensor 100, the portion of the piezoelectric substrate 111 that is clamped by the supporting member 101 is not polarized. Consequently, in the piezoelectric substrate 111, even in the case where the temperature of the atmosphere in which the acceleration sensor 100 is arranged increases, it is not likely that charge will be generated by the pyroelectric effect. Therefore, with the acceleration sensor 100, the acceleration can be detected with high accuracy regardless of a change in the temperature of the atmosphere in which the acceleration sensor 100 is arranged.
The following method is described in Patent Document 1 as a method of manufacturing the above-described acceleration sensor 100. First, for a plurality of first electrodes 112a, portions of electrodes 112a1 and 112a2 to be formed on the surfaces of the piezoelectric substrate 111 other than the portions that will be located in the clamped portion are formed and the second electrode 112b is formed. Next, the piezoelectric substrate 111 is polarized. A method is then described for forming the remaining portions of the electrodes 112a1 and 112a2.
Patent Document 1: WO 2007/132588 A1
However, in the acceleration sensor 100, it is necessary to divide the forming of the electrodes 112a1 and 112a2 into two steps. Consequently, there is a problem in that the manufacturing method is complex.
In addition, from the viewpoint of making the sensitivity of the acceleration sensor 100 high, it is preferable to make the portion of the piezoelectric substrate 111 that is polarized large. Accordingly, it is preferable that polarization be performed in a state where the electrodes have been formed over the entirety of the piezoelectric substrate 111 except for the clamped portion of the piezoelectric substrate 111. However, in this case if the accuracy with which the electrodes are formed is low or the accuracy with which the piezoelectric element 110 is combined with the supporting member 101 is low, part of the portion of the piezoelectric substrate 111 that is polarized may be clamped by the supporting member 101. Therefore, there is a problem in that manufacture of the acceleration sensor 100 is difficult.